1. Technical Field
The invention relates to precision optical devices such as astronomical reflecting telescopes with many reflecting or refracting surfaces whose deformations are to be compensated in order to maximize image quality. In particular, the invention relates to a feedback control system which computes in a single linear computation the exact set of actuator commands which minimize the total wavefront error at the telescope objective (output) for a given set of measured deformations of the reflecting or refracting surfaces of the optical device.
2. Background Art
Image quality in powerful optical devices such as interstellar reflecting telescopes is impaired by deformation of the reflecting and refracting optical surfaces in the telescope. Deformation of the optical surfaces due, for example, to temperature variations, vibration, etc., is unavoidable, and must be compensated if image quality is to be maximized. Such compensation ideally should be performed quickly in real time to permit rapid repetitive compensation in order to maintain maximum image quality at all times through varying conditions.
Alignment of a complex optical system is a related problem. U.S. Pat. Nos. 4,471,447 and 4,471,448 to Williams et al. and Williams, respectively, disclose a method for aligning a complex optical system in which the desired adjustment of all of the controlled optical surfaces is computed using a numerical regression estimation technique. In essence, this technique perturbs the alignment parameters in a model of the complex optical system in successive trial-and-error attempts to degrade the model and duplicate the distortion actually observed in the (objective) output image of the optical system. After many such attempts, the observed distortion is ultimately duplicated in the model, and the values to which the alignment parameters of the model have been last perturbed are used to calculate the alignment errors in the controlled optical surfaces of the complex optical systems. The alignment errors are then corrected.
The problem with such a technique is that the numerical regression process is so time-consuming that its use for anything other than initial optical alignment is impractical. During operation, the corrections would be performed so slowly using such a technique that they would not keep up with the continuing perturbations of the optical surfaces and in essence would be of no practical effect on image quality. Thus, it has not seemed possible to compensate for deformation of the optical surfaces in a complex optical system in real time.
A related problem is that the regressive estimation approach of such a technique by definition does not yield the exact answer after a limited number of iterations, and therefore provides inexact compensation for the deformation or movement of the optical surfaces in a complex optical system.
Accordingly, it is an object of the invention to provide a feedback control system for maintaining optical alignment in a complex optical system in real time.
It is a further object of the invention to perform a single rapid computation of the exact alignment corrections for all controlled optical elements in the optical system to minimize the total wavefront error at the objective (output) of the optical system.